When tungsten ores such as scheelite, wolframite, or hubnerite are digested to recover the tungsten, elements other than tungsten present in the ores are also removed. For instance, most ore types contain molybdenum, silica, phosphorus, arsenic, antimony, and some tin. If these elements are allowed to remain with the tungsten, they will in later processing steps cause high losses of tungsten by formation of heteropoly tungstates which are soluble. These heteroploy tungstates are also soluble in organic solvents. In organic/aqueous solvent extraction processing, this results in formation of a third phase or slow separation of the organic and aqueous phases. For all practical purposes, the silicon, arsenic, and phosphorus must be removed if tungsten is to be recovered with little loss.
To remove these elements, the industry uses a modified version of the analytical procedure. This is the formation of ammonium magnesium arsentate or phosphate, which can be filtered out of the tungsten solution. The caustic digestion of tungsten ores produces a solution of sodium tungstate, sodium molybdate, sodium phosphate, sodium silicate, and sodium arsenate. To this solution is added ammonium hydroxide, or ammonium chloride and magnesium sulfate or magnesium cloride. The pH of this solution is about 12.0 and is lowered to from about 9.0 to about 9.5. This causes precipitation of the silicon as silicon dioxide, and arsenic and phosphorus as their respective ammonium salts. These are all filtered off. There are significant amounts of tungsten entrapped or occluded in this precipitate also. This precipitate can contain by weight up to about 25% tungsten, about 5% arsenic, from about 1.7% to about 2.0% phosphorus, about 10.0% silicon, and about 10% magnesium. Such precipitates present a disposal problem because the arsenic content makes it a toxic material.
One present method for removing some of the tungsten from this type of material is simply by washing with water, However, only the soluble tungsten is removed in this way. The insoluble tungsten remains in the material and the material is still toxic.
Another method for removing tungsten from this type of material is digesting the material in strong sodium hydroxide. This removes the tungsten but also removes a significant portion of the arsenic and phosphorus.